1. Field of the Invention
The present invention relates to organic materials and a photoelectric conversion element using the same.
2. Description of the Related Art
In recent years, importance of solar cells has been increasing as an alternative energy to fossil fuel and as a measure against global warming. However, existing solar cells typified by silicon solar cells are costly at present, which is a factor of preventing them from being widely used. Therefore, research and development on various low-cost solar cells have been advancing. In particular, one example thereof is an organic thin film solar cell. The organic thin film solar cell is lightweight, low-priced, and easily enlarged in its surface area. Thus, expectations for practical applications have been increased. However, the photoelectric conversion efficiency of the organic thin film solar cell is still ½ or less that of the silicone-based solar cells.
A photoelectric conversion layer of the organic thin film solar cell includes a p-type semiconductor and a n-type semiconductor. Excitons generated in the p-type semiconductor and/or the n-type semiconductor by light absorption are dispersed to a pn interface, and then, are separated into electrons and holes at the pn interface. Next, the separated electrons and holes are each transferred to a trapping electrode, and are taken out to an external circuit. However, organic materials have an exciton diffusion length of about only several nanometers, and a diffusion length of the separated electric charge is also much inferior to silicon or the like although it cannot flatly be said since the diffusion length is greatly influenced by its crystalline state.
A bulk heterojunction structure which is disorderly formed by mixing the p-type semiconductor material and the n-type semiconductor material, or a regular bulk heterojunction structure which has order to ensure a charge transport path, or the like is one of the structures which solve the problems such as a diffusion length of the exciton and charge transportation, and are indispensable for improvement in photoelectric conversion efficiency. In the bulk heterojunction structure, the pn interface is disposed within a range of several nanometers from a place where the excitons are generated, which improves exciton separation efficiency. Moreover, by ensuring the charge transport path, the electric charge can be transferred to each of the electrode without binding them each other during transportation, and thus, a large amount of electrical current can be taken out to an external circuit.
Regarding organic materials mainly used at present, a lot of conjugated polymers have been reported as the p-type semiconductor material. The conjugated polymers easily form a bulk heterojunction structure, which is excellent in charge separation ability and charge transport ability. However, the conjugated polymers have a problem in that it is difficult to obtain high open circuit voltage since their ionization potential, which is a material property correlated to the open end voltage, is low due to extended conjugation thereof. Moreover, they are polymers and have a molecular weight distribution, which causes a problem in that stable materials are difficult to obtain during production.
In order to solve the problems, organic, low-molecular-weight, p-type semiconductor materials that can be formed into an element by a coating method, which needs no vacuum process, have been reported.
The present inventor has previously reported a photoelectric conversion element containing a diketopyrrolopyrrole derivative that is a low-molecular-weight organic material and has a certain structure (refer to “Chem. Mater., 2013, 25 (12), 2549-2556”). Moreover, the present inventor has also reported a diketopyrrolopyrrole derivative containing a benzodithiophene derivative (refer to “ACS Appl. Mater. Interfaces, 2013, 5 (6), 2033-2039”).
However, the former has a high ionization potential and a high open circuit voltage, but it can absorb light of short wavelengths, and thus is insufficient in photoelectric conversion efficiency. Meanwhile, the latter can absorb light of relatively longer wavelengths, but it is poor in aggregated structure responsible for charge transport path and charge separation, and thus is still insufficient in photoelectric conversion efficiency.
Thus, it has been desired to provide a novel organic material which has a high open circuit voltage, can absorb light of a wide wavelength range, and is excellent in charge transport ability, when it is applied to an organic thin film solar cell system.